辛苦老师帮忙看一下

来源：4-14 项目作业

小雨不大点

2021-04-23 21:51:13

相关代码：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <input type="text" class='num1'>
    <select name="operation" class="oper">
        <option value="+">+</option>
        <option value="-">-</option>
        <option value="*">*</option>
        <option value="/">/</option>
    </select>
    <input type="text" class='num2'>
    <input type="button" value='=' class='btn'>
    <input type="text" class='num3'>
<script>
    var num1=document.getElementsByClassName('num1')[0];
    var num2=document.getElementsByClassName('num2')[0];
    var num3=document.getElementsByClassName('num3')[0];
    var btn=document.getElementsByClassName('btn')[0];
    var oper=document.getElementsByClassName('oper')[0];
    var n1=Number(num1.value);
    var n2=Number(num2.value);
    var n3;
    btn.onclick=function(){
        var n1=Number(num1.value);
        var n2=Number(num2.value);
        if(oper.value=='+'){
            n3=n1+n2;
            console.log(n3);
            console.log(num3);
            }else if(oper.value=='-'){
                n3=n1-n2; 
                console.log(n3);
                console.log(num3);
            }else if(oper.value=='*'){
                n3=n1*n2; 
                console.log(n3);
                console.log(num3);
            }else{
                n3=n1/n2; 
                console.log(n3);
                console.log(num3);
            }
        if(isNaN(n3)){
            num3.placeholder='请输入数字';
            console.log('3424');
        }else{ 
            num3.placeholder=n3;
        }

    }
</script>

</body>
</html>